Review/redesign/refactor ScheduledExecutor

[sighphyre]

Implementation of bootstrapping has lead to some subtle changes to the ScheduledExecutor class. There's some discussion around how this needs to work and what the code needs to look like. Ideally, we need to define the behaviour we'd like to see from this class before we change code.

This is especially important to decide so that we can finalise how async intialisation works (which we partially support at the moment) and will be important if we decide we need to implement the "block until ready" that some of the other SDKs support. Both of these would be surprising, breaking changes from a consumer perspective so deciding on how ScheduledExecutor needs to work is important

[stale]

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